# Deneb -- Honest Validator

**Notice**: This document is a work-in-progress for researchers and implementers.

## Table of contents

<!-- TOC -->
<!-- START doctoc generated TOC please keep comment here to allow auto update -->
<!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->

- [Introduction](#introduction)
- [Prerequisites](#prerequisites)
- [Constants](#constants)
  - [Misc](#misc)
- [Helpers](#helpers)
  - [`BlobsBundle`](#blobsbundle)
  - [Modified `GetPayloadResponse`](#modified-getpayloadresponse)
- [Protocol](#protocol)
  - [`ExecutionEngine`](#executionengine)
    - [Modified `get_payload`](#modified-get_payload)
- [Beacon chain responsibilities](#beacon-chain-responsibilities)
  - [Block and sidecar proposal](#block-and-sidecar-proposal)
    - [Constructing the `BeaconBlockBody`](#constructing-the-beaconblockbody)
      - [Blob KZG commitments](#blob-kzg-commitments)
    - [Constructing the `SignedBlobSidecar`s](#constructing-the-signedblobsidecars)
      - [Sidecar](#sidecar)

<!-- END doctoc generated TOC please keep comment here to allow auto update -->
<!-- /TOC -->

## Introduction

This document represents the changes to be made in the code of an "honest validator" to implement Deneb.

## Prerequisites

This document is an extension of the [Capella -- Honest Validator](../capella/validator.md) guide.
All behaviors and definitions defined in this document, and documents it extends, carry over unless explicitly noted or overridden.

All terminology, constants, functions, and protocol mechanics defined in the updated [Beacon Chain doc of Deneb](./beacon-chain.md) are requisite for this document and used throughout.
Please see related Beacon Chain doc before continuing and use them as a reference throughout.

## Constants

### Misc

| Name | Value | Unit |
| - | - | :-: |
| `BLOB_SIDECAR_SUBNET_COUNT` | `4` | The number of blob sidecar subnets used in the gossipsub protocol. |

## Helpers

### `BlobsBundle`

```python
@dataclass
class BlobsBundle(object):
    commitments: Sequence[KZGCommitment]
    proofs: Sequence[KZGProof]
    blobs: Sequence[Blob]
```

### Modified `GetPayloadResponse`

```python
@dataclass
class GetPayloadResponse(object):
    execution_payload: ExecutionPayload
    block_value: uint256
    blobs_bundle: BlobsBundle
```

## Protocol

### `ExecutionEngine`

#### Modified `get_payload`

Given the `payload_id`, `get_payload` returns the most recent version of the execution payload that
has been built since the corresponding call to `notify_forkchoice_updated` method.

```python
def get_payload(self: ExecutionEngine, payload_id: PayloadId) -> GetPayloadResponse:
    """
    Return ExecutionPayload, uint256, BlobsBundle objects.
    """
    # pylint: disable=unused-argument
    ...
```

## Beacon chain responsibilities

All validator responsibilities remain unchanged other than those noted below.

### Block and sidecar proposal

#### Constructing the `BeaconBlockBody`

##### Blob KZG commitments

1. After retrieving the execution payload from the execution engine as specified in Capella,
use the `payload_id` to retrieve `blobs`, `blob_kzg_commitments`, and `blob_kzg_proofs`
via `get_payload(payload_id).blobs_bundle`.
2. Validate `blobs` and `blob_kzg_commitments`:

```python
def validate_blobs_and_kzg_commitments(blobs: Sequence[Blob],
                                       blob_kzg_commitments: Sequence[KZGCommitment],
                                       blob_kzg_proofs: Sequence[KZGProof]) -> None:
    # Optionally sanity-check that the KZG commitments match the blobs (as produced by the execution engine)
    assert len(blob_kzg_commitments) == len(blobs) == len(blob_kzg_proofs)
    assert verify_blob_kzg_proof_batch(blobs, blob_kzg_commitments, blob_kzg_proofs)
```

3. If valid, set `block.body.blob_kzg_commitments = blob_kzg_commitments`.

#### Constructing the `SignedBlobSidecar`s

To construct a `SignedBlobSidecar`, a `signed_blob_sidecar` is defined with the necessary context for block and sidecar proposal.

##### Sidecar

Blobs associated with a block are packaged into sidecar objects for distribution to the network.

Each `sidecar` is obtained from:
```python
def get_blob_sidecars(block: BeaconBlock,
                      blobs: Sequence[Blob],
                      blob_kzg_proofs: Sequence[KZGProof]) -> Sequence[BlobSidecar]:
    return [
        BlobSidecar(
            block_root=hash_tree_root(block),
            index=index,
            slot=block.slot,
            block_parent_root=block.parent_root,
            blob=blob,
            kzg_commitment=block.body.blob_kzg_commitments[index],
            kzg_proof=blob_kzg_proofs[index],
        )
        for index, blob in enumerate(blobs)
    ]

```

Then for each sidecar, `signed_sidecar = SignedBlobSidecar(message=sidecar, signature=signature)` is constructed and published to the associated sidecar topic, the `blob_sidecar_{subnet_id}` pubsub topic.

`signature` is obtained from:

```python
def get_blob_sidecar_signature(state: BeaconState,
                               sidecar: BlobSidecar,
                               privkey: int) -> BLSSignature:
    domain = get_domain(state, DOMAIN_BLOB_SIDECAR, compute_epoch_at_slot(sidecar.slot))
    signing_root = compute_signing_root(sidecar, domain)
    return bls.Sign(privkey, signing_root)
```

The `subnet_id` for the `signed_sidecar` is calculated with:
- Let `blob_index = signed_sidecar.message.index`.
- Let `subnet_id = compute_subnet_for_blob_sidecar(blob_index)`.

```python
def compute_subnet_for_blob_sidecar(blob_index: BlobIndex) -> SubnetID:
    return SubnetID(blob_index % BLOB_SIDECAR_SUBNET_COUNT)
```

After publishing the peers on the network may request the sidecar through sync-requests, or a local user may be interested.

The validator MUST hold on to sidecars for `MIN_EPOCHS_FOR_BLOB_SIDECARS_REQUESTS` epochs and serve when capable,
to ensure the data-availability of these blobs throughout the network.

After `MIN_EPOCHS_FOR_BLOB_SIDECARS_REQUESTS` nodes MAY prune the sidecars and/or stop serving them.